In gas turbine aeroengines it is necessary to maintain the engine fuel and the engine lubricating oil within predetermined ranges of temperature so that the fuel and oil temperatures are suitable throughout the whole range of engine operation.
It is common practice to use the engine fuel as a heat sink to cool the engine lubricating oil, using a fuel cooled oil cooler heat exchanger. However, in the operation of certain large passenger aircraft it has been found to be necessary for the engine lubricating oil to be independently heated or cooled to avoid the fuel temperature straying outside acceptable limits.